The morphodynamic changes occurring during growth were evaluated in the eel (Anguilla anguilla L.) heart. Using an in vitro working heart preparation, cardiac performance of small (body mass 96.76 +/- 27.49 g; mean +/- s.d.) and large (body mass 656 +/- 12 g; mean +/- s.d.) eels was compared under basal conditions and under loading (i.e. preload and afterload) challenges. A parallel morphometric evaluation of the ventricle was made using light and transmission electron microscope images. The small eel hearts show a basal cardiac output lower than their large counterparts (heart rate fh, 38.93 +/- 2.82 and 52.7 +/- 1.8 beats min(-1), respectively; stroke volume Vs, 0.27 +/- 0.017 and 0.37 +/- 0.016 ml kg(-1), respectively; means +/- s.e.m.). The two groups show similar responses at increasing preload, but differ remarkably at increasing afterload. Small eel hearts decreased Vs at afterload greater than 3 kPa, in contrast to larger hearts, which maintained constant Vs up to 6 kPa. These changes in mechanical performance are related to structural differences. Compared with the small eels, the large eels show an increase in the compacta thickness and in the diameter of the trabeculae in the spongiosa, together with reduction of the lacunary spaces. The increased compacta thickness is attained by enlargements of both the muscular and vascular compartments and reduction of the interstitium; consequently, this layer appears more compacted. Both compacta and spongiosa show higher number of myocytes together with reduced cross-sectional area and myofibrillar compartment. The compacta also shows an increased mitochondrial compartment. Our results document a cardiac morphodynamic remodelling in the growing eel.